Bake bread pieces until dried and slightly colored, stirring occasionally, about 1 hour 15 minutes. Cool completely. Using potato masher, coarsely crush bread on sheet. DO AHEAD Can be made 3 days ahead. Cover and chill. Rewarm in 350°F oven 4 to 5 minutes before using.

VINAIGRETTE

Place first 3 ingredients in medium bowl. Gradually whisk in oil. Season vinaigrette to taste with salt and pepper. DO AHEAD Can be made 3 days ahead. Cover; chill. Bring to room temperature and rewhisk before using.

LEEKS

Position rack in center of oven and preheat to 450°F. Cut leeks in half lengthwise. Rinse under cold water to remove any grit between layers; drain on paper towels. Arrange leeks, cut side down, in single layer in 13x9x2-inch glass baking dish. Pour broth over; drizzle with oil. Top with thyme sprigs and lemon slices. Cover dish tightly with plastic wrap, then foil.